1. Field of the Invention
The invention disclosed herein relates to oil field exploration and, in particular, to avoiding collisions between wells drilled during exploration.
2. Description of the Related Art
Unplanned collisions between oilwells can have catastrophic results. The industry therefore has an interest in developing risk assessment tools, including well-founded means for estimating the probability of such collisions.
Recent interest has focused on the development of improved models for describing survey accuracy and quality control of survey data to assure compliance with these models. The survey data along with the appropriate error models provide a basis for estimating the probability of collision.
In spite of pioneering attempts to express the complex problem in a simple form, most operators today still rely on rule-of-thumb methods with little mathematical foundation. For example, a clearance factor or separation factor is widely used as an indicator of collision probability. Such factors basically involve a ratio of well separation to positional uncertainty. There are many different implementations, none of which bears a strong mathematical correlation to collision probability. There is a better method with recent variations used for low-risk wells. This method and the variations thereof assume straight non-parallel wells, in which case the probability of collision depends only on the nominal separation and the positional uncertainties in the direction normal to the two wellpaths. Probability can be estimated by integration of a one-dimensional probability density function. When applied to points at the closest approach of two straight non-parallel wells, this method can give a meaningful estimate of the overall collision probability, under the assumption that the relative uncertainty does not change significantly over the intervals of interest. However, this method is unsuitable for evaluating collision risk over short intervals or between curved wellpaths or where the relative uncertainty cannot be assumed constant over the intervals of interest.
Methods which solve the problem by integration of two-dimensional (2D) or three-dimensional (3D) probability density functions are known. While sometimes less restrictive than the one-dimensional (1D) integration, these methods do not completely represent the general problem. A useful test of a method is whether it produces accurate results for both parallel and non-parallel wells, for straight or curved wellpaths, and for intervals of wells whose relative uncertainty may not be constant.
It must be recognized that a numerical estimate of collision probability is no better than the data from which it is derived. While a sound mathematical computation of probability is helpful, it also requires accurate knowledge of the magnitude and distribution of survey errors. Many, perhaps most, unplanned collisions result from human failures causing gross well positioning errors beyond the modeled error budget.
Therefore, what are needed are improved techniques for avoiding collision with an existing well while drilling another well. Preferably, the techniques make use of improved data and produce accurate results both for non-parallel wells and for parallel wells, which may be straight or curved with constant or varying relative spatial uncertainty.